khmer.readthedocs.org points to 2.1.1

[ctb]

...instead of 2.1.2. We should include updating the readthedocs pointer in the release notes!

[standage]

For a while the default was stable, which should automagically reset whenever we issue a new stable release. That would have worked fine, except we had a conflict with a legacy branch called stable. I opened an issue with the ReadTheDocs folk, and they think they have solved the issue, but I haven't had a chance to check yet.